Which of these numbers is prime? ${35,\ 69,\ 79,\ 87,\ 98}$
Explanation: A prime number has exactly two factors: 1 and itself. The factors of 35 are 1, 5, 7, and 35. The factors of 69 are 1, 3, 23, and 69. The factors of 79 are 1 and 79. The factors of 87 are 1, 3, 29, and 87. The factors of 98 are 1, 2, 7, 14, 49, and 98. Thus, 79 is a prime number.